Nitrided vanadium, columbium and tantalum base alloys

ABSTRACT

A novel group of nitrided alloys having excellent wear and abrasion resistance containing as their major constituent at least two metals of the group columbium, tantalum and vanadium alloyed with titanium and/or zirconium. Such alloys are fabricated to shape and are then nitrided to produce high-surface hardness thereon. The nitrided alloys have utility as cutting tool materials and other areas of use where excellent wear and abrasion resistance are desired.

United States Patent Van Thyne et al.

[54] NITRIDED VANADIUM, COLUMBIUM AND TANTALUM BASE ALLOYS [72] Inventors: Ray J. Van Thyne, Oak Lawn; John J.

Rausch, Antioch, both of III [73] Assignee: Surface Technology Corporation, Stone Park, Ill.

[22] Filed: Mar. 4, 1970 [21 Appl. No.: 16,570

[52] US. Cl. ..l48/3l.5, 29/1825, 75/134, 75/174, 148/13.1, 148/203, 148/32, 148/34,

[51] Int. Cl ..C22c 27/00, C230 1 1/14 [58] Field ofSearch ..75/134, 174, 205, 208, 224; 148/13.1, 20.3, 31.5, 34, 16.6, 39; 29/1822, 182.5

[56] References Cited UNITED STATES PATENTS 9/1935 Austin ..148/20.3 5/1937 Kelly ....75/174 X 8/1939 Van Note ....148/20.3 8/1957 Wyatt et al ..148/133 X [151 3,642,546 1 Feb. 15, 1972 6/1962 Berger et al ..75/1 77 X 12/1966 Gilliland ..75/l74 X OTHER PUBLICATIONS IIT Research Institute, Reports IR-718-7 (I) and (II) Aug 22,1967 & Sept. 1967 W Primary Examiner-Charles N. Lovell Attorney-Albert Siegel [5 7] ABSTRACT 5 Claims, No Drawings BACKGROUND OF THE INVENTION Our invention relates to a novel group of nitrided ternary alloys consisting of a principal or major constituent at least two metals of the group columbium, tantalum and vanadium alloyed with titanium and/or zirconium in amounts of percentages by weight as is hereinafter set forth. We have found that such nitrided alloys demonstrate high surface hardness without accompanying brittleness and offer promise for applications requiring wear and abrasion resistance. We would note that the present application is directed to the use of at least two metals of the group columbium, tantalum and vanadium in combination with titanium and/or zirconium.

It is well known that titanium can be nitrided to form a hard surface layer thereon but such material shows a chipping propensity due to brittleness. In the practice of our invention, such brittleness is avoided by specific alloying as taught herein prior to nitriding. As shown subsequently herein, the alloying elements present in typical commercially available titanium alloys do not produce the improvement as herein taught and nitrided commercial titanium alloys show chipping similar to nitrided titanium.

The nitriding of titanium-rich alloys, i.e., containing about 90 percent titanium has been studied previously (for example, see E. Mitchell and P. .I. Brotherton, .1. Institute of Metals Vol. 93 (1964) P. 381). Others have investigated the nitriding of hafnium-base alloys (F. Holtz, et al., U.S. Air Force Report lR-7l8- (II), 1967); molybdenum alloys (U.S. Pat. No. 3,161,949); and tungsten alloys (D. .I. Iden and L. Himmel, Acta Met, vol. 17 (1969) P. 1483). The treatment oftantalum and certain unspecified tantalum base alloys with air or nitrogen or oxygen is disclosed in U.S. Pat. No. 2,170,844 and the nitriding of columbium is discussed in the paper by R. P. Elliott and S. Komjathy, AIME Metallurgical Society Conference, Vol. 10, 1969, P. 367.

In our copending patent applications Wear Resistant Materials Ser. No. 755,658 and Wear and Abrasion Resistant Materials" Ser. No. 755,622, we have disclosed and claimed certain nitrided three through seven metal alloy systems which are characterized by excellent cutting performance. Counterparts to said U.S. applications have now been issued as Belgium Pat. Nos. 720,398 and 720,399. Such applications and Belgium patents are directed to nitrided alloys containing:

a. one or more metals of the and vanadium;

b. one or both of the metals molybdenum and tungsten; and

c. titanium and/or zirconium, in certain percentages by weight as is therein set forth.

We have now discovered that certain nitrided ternary alloys such as Cb-Ta-Ti offer promise for applications requiring abrasion resistance. Although the overall performance of the present nitrided ternary alloys is somewhat lessened compared to the ternary alloys containing tungsten and/or molybdenum in terms of cutting properties, the present ternary alloys find utility because, for example, the unnitrided alloys with no molybdenum or tungsten have somewhat improved cold fabricability which is useful for the preparation of the most intricate parts prior to nitriding (although it should be recognized that certain of the unnitrided ternary alloys with molybdenum or tungsten also possess good cold forming characteristics).

Accordingly, a principal object of our invention is to provide novel nitrided alloys containing at least two metals of the group columbium, tantalum, and vanadium with titanium and/or zirconium having excellent wear and abrasion resistance and utility as cutting tool materials.

This and other objects, features and advantages of our invention will become apparent to those skilled in this particular art from the following detailed disclosure thereof.

In order to best understand our invention reference should be had to the experimental procedures which we employed.

group columbium, tantalum 2 EXPERIMENTAL PROCEDURES In our experimental work a series of alloys were melted under an inert atmosphere in a nonconsumable electrode arc furnace using a water-cooled, copper hearth. High purity materials (greater than 99.5 percent) were used for the alloy charges and generally weighed about 50 grams. These procedures are, of course, quite well known to those skilled in the art.

The alloys were cut into specimens approximately oneeighth inch thick and reacted in nitrogen at atmospheric pressure. The resulting thickness and mircohardnes'ses of the various reaction zone or layers were determine using standard metallographic techniques. Several tests were used to evaluate the strength and toughness of these materials for potential use in abrasive wear or metal cutting applications.

The metal cutting tests were performed using the nitrided materials as tool inserts %X% /a inch having an 0.030 inch nose radius which was used as a section of the cutting surface. Such radii were ground on the specimens prior to nitriding.

The alloy samples thus prepared were subsequently nitrided. For nitriding we used a cold wall furnace employing a molybdenum heating element and radiation shields with the furnace being evacuated to 5 microns pressure and flushed with nitrogen prior to heating. Temperatures were measured with an optical pyrometer, namely a Leeds and Northrop Optical Pyrometer, catalog number 862, sighting on an unnitrided molybdenum heating element which completely surrounded the specimens. The temperature given herein are corrected from this source. We used a correction factor determined by using a tungsten-rhenium thermocouple in conjunction with the sightings of the aforesaid optical pyrometer.

Following nitrided sample preparation, lathe turning tests were run on AISI 4340 steel having a hardness of Rockwell C (Re), 44. A feed rate of 0.005 inch per revolution and depth of cut of 0.050 inches were used. A standard negative rake tool holder was employed with a 5 back rake and a 15 side cutting edge angle.

Our principal criterion in determining whether the present nitrided materials passed or failed and thus whether they are useful or not for purposes of the present invention was the ability to remove 2 cubic inches of hardened steel at a speed of at least 400 s.f.m. (surface feet per minute). Furthermore, we evaluated the toughness and chipping resistance by using a conical diamond hardness indentation (standard Rockwell A scale60 kg. load) and evaluating whether chipping occurred around the hardness impression using a 10 eyepiece magnifier. All of our present materials pass the cutting test at 400 or 750 s.f.m. and do not exhibit significant chipping around the Ra hardness indentations. Of course our nitrided materials have substantial applicability in wear and abrasion applications other than cutting, but the cutting test is a standardized and readily producible test.

DESCRIPTION OF THE INVENTION We have discovered a novel group of nitrided alloys characterized by high surface hardness and useful wear resistance. These materials are formed when alloys within our prescribed compositional ranges as hereinafter taught are reacted with nitrogen or an environment which is nitriding to the alloys at elevated temperatures. The hardening reaction is typical of the internal oxidation or nitridation techniques well known in the art. The volume of hard constituents formed at the surface is high in our materials. As nitrogen diffuses inwardly, there is developed a variety of nitrided phases. Nitride formation lessens inward from the surface and this grading contributes to the excellent thermal and mechanical shock resistance of the material. Our materials are characterized by being graded with the degree of nitride formation lessening as one moves inward from the surface.

In the present application, the nitrided alloys contain from 12 percent to 35 percent titanium and/or zirconium-balance two or more of the group columbium, tantalum and vanadium.

These elements readily interalloy and the present nitrided materials containing two or more elements of the group columbium, tantalum, or vanadium with titanium and/or zirconium demonstrate desirable combination of properties, i.e., high hardness and wear resistance along with absence of chipping propensity when the nitrided alloy compositions are within the prescribed ranges. In fact, we find advantages and improvements when more than one element of the group columbium, tantalum, or vanadium is present. Within the scope of our investigations, we have found Cb-Ta-Ti alloys that represented a combination of ease of unnitrided alloy preparation and cutting performance after nitriding not found in the respective binary systems. Also, high cutting performance coupled with lower nitriding temperature is found with V-Cb-Ti material.

Unalloyed titanium was nitrided for 2 hours at 2,250 and 2,850 F. Both specimens showed chipping at the edges as a result of cooling from the nitriding temperature and Ra hardness indentations resulted in serious chipping all around the impressions. These materials fail immediately in the 400 s.f.m. cutting test. The possibility that the alloying elements present in commercial titanium alloys might result in an improved nitrided material was examined. Two of the more highly alloyed commercial materials, Ti-6Al-4V and Ti-3A1-l3V-l lCr were nitrided at 2,850 F. for 2 hours. Chipping of the edges occurred during cooling and metallographic examination confirmed that similar to unalloyed titanium, thick continuous nitrided surface layers existed. The Ti-6Al4V alloy exhibited a 10 mil layer and the Ti-3Al-l3V-llCr alloy showed a 2 to 5 mil layer with a serrated boundary between the nitrided layer and the substrate. Both of these materials failed immediately in cutting at 400 s.f.m. and are excluded from our invention. Similarly, nitrided unalloyed columbium, tantalum, and vanadium fail.

Cutting test findings for a range of nitrided Ta-Cb-Ti alloys are shown in Table l.

TABLEI Nitriding Composition Treatment Cutting (weight 7U Hr Test Unalloyed Cb 3250 2 F Unalloyed Ta 3250 2 F Unalloyed V 2850 2 F 4STa-45Cb-lOTi 3250 2 P 60 Ta-ZOCb-ZOTi 3250 2 P 40 Ta40 Cb-Ti 3250 2 P 20 Til-60 Cb-20 Ti 3250 2 P 45 Tal5Cb-40Ti 3250 2 P Ta- 30 Cb-40 Ti 3250 2 P 20 Ta -20 Cb-oO Ti 3250 2 F 40 Ta 40 Ch 20 Zr 3250 2 P ISTa-lSCb-70Zr 3050 2 F 60 Cb- 20 \"2.0 Ti 2450 4 P 20 Cb 60 V 20 Ti 2000 2 l5Cb-l5V-70Ti 3050 2 F 50 Cb- 20V-30 Zr 2450 2 P 60 Tu 20 V 20 Ti 2450 4 P 60 Ta 20 V 20 Zr 2450 2 P Ta-30Cbl0V 25 Ti 2650 2 P 30Ta-3OCb- 10V- l5Ti-l5Zr 2650 2 P P Passremoves 2 cu. in. ot'4340 steel workpiece at 400 or 750 s.f.m. F Fails at 400 s.f.m.

Nitrided 45Ta45Cb-l0Ti shows considerable chipping around Ra impressions and is excluded from our invention. At the 20 percent titanium level, three nitrided alloys 60Ta-20 Cb-20Ti, 40Ta-40Cb-20Ti, and 20Ta-60Cb-20Ti all pass at 400 s.f.m. and in fact cut at 750 s.f.m. These materials do not show chipping around the Ra hardness impressions and are included within our invention.

Two nitrided alloys at the 40 percent titanium level were evaluated. Such alloys are excluded from the scope hereof.

(Int

The nitrided alloy 2OTa2OCb-6OTi fails in cutting at 400 s.f.m., shows pronounced chipping around indentations, and is excluded from our invention. Thus, the nitrided Ta-Cb-Ti alloys falling within the scope of our invention contain 12 percent to 35 percent titanium.

Similar trends were noted with zirconium additions. The nitrided 4OTa-4OCb-2OZr alloy cut at 400 and 750 s.f.m. and is included within our invention. However, nitrided l5Ta-l5 Cb-Zr failed to cut at 400 s.f.m. and is excluded from our invention,

Another system included herein is Cb-V-Ti. cent titanium level, nitrided 6OCb-20V-20Ti and 2OCb-6O V20 Ti pass our test criteria and are included within our invention. As shown in the Table, the alloy 20Cb-6OV-20Ti was nitrided at a comparatively low temperature, but still cut at 750 s.f.m with this treatment. The nitrided alloy l5Cb-l5 70Ti chips with Ra indentations to the extent that a hardness reading could not be taken. The material fails in cutting at 400 s.f.m. and even fails immediately in cutting at 200 s.f.m. This composition is excluded from our invention. Nitrided 50Cb-20V-30Zr passes our test criteria and such materials are included within our invention.

Alloys tested involving a Ta-V base included 60Ta-20 V2OTi and 6OTa2OV-2OZr. All such nitrided materials pass the test criteria and are included within our invention.

We have also demonstrated good performance for alloys containing more than 3 constituents. The nitrided alloys 35Ta30Cb-l0V-25Ti and 30Ta-3OCb-lOV-l 5Til 5Zr pass and nitrided materials containing all three elements of the group columbium, tantalum, and vanadium with titanium At the 20 perand/or zirconium are included within the scope of the our invention if the range of combined titanium and zirconium is 12 to 35 percent.

A variety of nitrogen containing environments can be used to produce similar hardened materials. However, upon reacting in a much lower nitrogen potential environment, the effect of lowered nitrogen availability is observed and a somewhat modified reaction product may be obtained. For example, similar size samples of 40Ta-40Cb-20Ti were nitrided at 3050 F. for 2 hours in nitrogen and argon-0.l percent nitrogen. The nitrogen pickup was somewhat reduced in the latter environment.

Since our surface reacted composites are in a thermodynamically mestastable condition, those skilled in the art will realize that a variety of heat treatments, including multiple and sequential treatments, can be used to modify the reaction product and resulting properties whether performed as part of the overall nitriding reaction or as separate treatments. The materials can also be nitrided at higher temperatures (and times) that normally would produce some embrittlement and then subsequently annealed in inert gas or various partial pressures of nitrogen as a tempering or drawing operation to improve toughness. This duplex treatment results in a deeper reaction product with the hardness-toughness relationship controlled by the tempering temperature and time.

In practicing the teachings of our invention, from the foregoing it should be borne in mind that nitriding times and temperatures are variable over a considerable range. Generally, for cutting tool uses we nitride for around 2 hours at the temperatures shown as useful in Table I.

We have modified our nitrided material by combining nitriding with oxidizing or boronizing. However, the amount of reaction with the other hardening agents must be limited, a majority of the weight pickup is due to nitriding, and these are essentially nitrided materials. The alloys may be preoxidized at a temperature where little reaction would occur with nitrogen alone and then subsequently nitrided. Also, the alloys may be reacted with a combined oxidizing and nitriding environment although the relative oxidizing potential must be low since for example in air the alloys will preferentially oxidize rather than nitride. A sample of 40Ta-4OCb-20Ti was nitrided at 3,050 F. for 2 hours and subsequently boronized at 2,650 F. for 4 hours. The structural features of such a material are very similar to the alloy only nitrided; the hardness grades inwardly and of the total weight pickup over 90 is due to nitriding. A smooth surface layer about 0.5 mil thick forms due to the boronizing treatment that is harder than the nitrided surface. Up to 25 percent of the nitrogen pickup, by weight, may be replaced by oxygen and/or boron.

The present useful alloys may also be produced by powder processing techniques. Also such alloys could be employed on another metal or alloy as a surface coating or cladding and with the proper selection, a highly ductile or essentially unreacted substrate can be obtained. For example, columbium and tantalum are much less reactive to nitrogen when used in conjunction with the alloys and molybdenum is essentially inert to nitrogen. Spraying and/or fusing the desired alloy onto the surface are among the various methods for depositing the ternary or more complex (Cb,Ta,V)-(Ti, Zr) alloys. Small other additions may be made to our alloys to enhance the coatability. A variety of direct deposition methods may be employed or alternate layers could be deposited followed by a diffusion annealing treatment. Another surface alloying procedure involves titanizing and/or zirconizing-for example the titanizing of a Cb-Ta alloy substrate. The nitrided material can be used as a mechanically locked insert or it can be bonded or joined by brazing, for example, to a substrate.

The high surface hardness of the nitrided alloys has been measured by 50 and 200 gram diamond pyramid microhardness traverses on metallographically polished cross sections. For the one-eighth inch thick materials falling within our invcntion the hardness measured at about 0.5 mil from the surface is in the range of 1,000 to 2,500 DPN and the hardness grades inwardly in a mostly continuous fashion. The nitrogen pickup is in excess of 1 mg. per sq. cm. for all of the examples shown in Table 1. However, the amount of nitrogen required for an equivalent surface hardness is substantially reduced when the material is used as a thin blade edge or sheet or as a thin coating or cladding. Also, such materials may be used for a wide variety of applications requiring wear and abrasion resistance where the requirement for surface hardness or depth of hardening may be less than that required for metal cutting. Thus, for certain applications, the nitrogen pickup might be 0.1 to 1 mg. sq. cm. ofsurface area.

We have also observed the excellent corrosion resistance of both the alloys and the nitrided alloys in strong acids, and

these materials could effectively be employed for applications requiring both corrosion and abrasion resistance. Both the alloys and the nitrided alloys possess good structural strength. Thus, the materials can be employed for applications involving wear resistance and structural properties (hardness, strength, stiffness, toughness) at room and elevated temperatures. Other useful properties of the nitrided materials include good electrical and thermal conductivity, high melting temperature, and thermal shock resistance.

Although the alloys receptive to nitriding can be produced by coating or surface alloying techniques, many uses involve the forming and machining of a homogeneous alloy. One of some applications, the utility would be enhanced by lapping, polishing, or other finishing operations after nitriding. The nitrided surface is quite hard but only a small amount of material removal is required to produce a highly finished surface.

The excellent cutting properties and wear resistance of the nitrided materials can be effectively employed with the other useful are: single point cutting tools, multiple point cutting tools (including rotary burrs, files, routers and saws), drills, taps, punches, dies for extrusion, drawing, and other forming o erations, armor gun barrel liners, impeller or fan blades, EDM (Electrical Discharge Machining) electrodes, spinnerets, guides (thread, wire, and other), knives, razor blades, scrapers, slitters, shears, forming rolls, grinding media, pulverizing hammers and rolls, capstans, needles, gages (thread, plug, and ring), bearings and bushings, nozzles, cylinder liners, tire studs, pump parts, mechanical seals such as rotary seals and valve components, engine components, brake plates, screens, feed screws, sprockets and chains, specialized electrical contacts, fluid protection tubes, crucibles, molds and casting dies, and a variety of parts used in corrosion-abrasion environments in the papermaking or petrochemical industries, for example.

It will be understood that various modifications and variations may be affected without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. A graded, nitrided material having excellent wear and abrasion resistant properties consisting essentially of: at least two metals of the group columbium, tantalum and vanadium alloyed with a metal selected from the group consisting of titanium and zirconium and mixtures thereof wherein a. the nitrogen weight pickup is at least 0.1 milligram per square centimeter; and

b. the group titanium, zirconium and mixtures thereof is present in amounts ranging from 12 percent to 35 percent.

2. The material as defined in claim 1 wherein the nitrogen weight pickup is at least 1 milligram per square centimeter.

3. The material as defined in claim 1 wherein the surface hardness thereof is at least 1,000 diamond pyramid numerals.

4. The material as defined in claim 1 wherein up to 25 percent of the nitrogen weight pickup is replaced by a material selected from the group consisting of oxygen and boron.

5. An article consisting essentially of a substrate member having a surface zone of a material defined in claim 1. 

2. The material as defined in claim 1 wherein the nitrogen weight pickup is at least 1 milligram per square centimeter.
 3. The material as defined in claim 1 wherein the surface hardness thereof is at least 1,000 diamond pyramid numerals.
 4. The material as defined in claim 1 wherein up to 25 percent of the nitrogen weight pickup is replaced by a material selected from the group consisting of oxygen and boron.
 5. An article consisting essentially of a substrate member having a surface zone of a material defined in claim
 1. 